1. Field of the Invention
The present invention pertains to the field of optoelectronics. The invention more particularly concerns a device which selectively controls the emission pattern of a vertical cavity surface emitting laser (VCSEL).
2. Discussion of the Background
VCSELs are key devices used in transceiver subassemblies for high-speed optical communication. The LED-like structure makes the VCSEL a better light source in optical transceiver modules than the edge emitting laser diode. The advantages of a VCSEL arise from its easy testing, easy packaging, and low power consumption. However, the emission pattern dependent high-speed optical waveforms produced by VCSELs can cause significant problems at high data transmission rates.
It has been determined that the modulation speed of light out of a VCSEL is a spatial function of its emission angle. FIG. 1 is a side view of a VCSEL 1. An axis of emission 5 is oriented normal or perpendicular to a surface plane 2 of the VCSEL 1. The surface plane 2 of the VCSEL 1 runs into and out of the paper of FIG. 1. In general, the larger the angle θ from perpendicular, the slower the modulation speed of the emitted light from the VCSEL 1. FIG. 1 shows that a fast pattern of light 3 is emitted from the VCSEL 1 along the axis of emission 5 and that a slow pattern of light 4 is emitted from the VCSEL 1 at the angle θ away from the axis of emission 5. The speed dependent emission pattern is symmetrical in all directions perpendicular to the axis of emission 5.
In low speed applications, such as in Gigabit Ethernet applications, where only one Gigabits per second of modulation is needed, the presence of fast and slow patterns of light is not an issue, since the slower light is fast enough to catch up to the electrical modulation. However, when pushing to higher speeds, such as two and one-half Gigabits per second and beyond, the light having the slow modulation has caused significant degradation in the eye-diagram quality. The resulting jitter and eye-closure of the performance of the VCSEL-based device in such applications has greatly reduced the achievable optical link distance.
Thus, there is a need to either eliminate the slow light pattern 4 or to separate the slow light pattern 4 from the fast light pattern 3 so as to increase the achievable optical link distance of VCSEL-based devices at high modulation speeds.